JP2001078083A - Exposure time controller for electronic still camera - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deterioration in the picture quality of a photographic picture by reducing the influence of noise to the photographic picture caused by dark current. SOLUTION: A temperature sensor 60 is provided in the neighborhood of a CCD 33 to measure the temperature of the CCD 33. A maximum exposure time in automatic exposure control is reduced with rise in the temperature of the CCD 33. If the maximum exposure time at 0 deg.C is t0, and it is assumed that 7<Th<11, then the maximum exposure time tmax at temperature T deg.C is given by tmax=t0×(1/2)T/Th, for example. When the value of the dark current generated at the photodiode of the CCD 33 is increased, the maximum exposure time is restricted to be reduced. Thus, the quantity of noise component occurring at a photographic picture is suppressed and the deterioration in picture quality is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for limiting the maximum value of an exposure time in an electronic still camera.

[0002]

2. Description of the Related Art Conventionally, as an electronic still camera, there has been known an electronic still camera capable of performing a long-time exposure of, for example, 10 seconds or more, such as bulb photography. An electronic still camera is provided with an image pickup device such as a CCD, and the image pickup device accumulates charges corresponding to a subject image formed on a light receiving surface. The charge accumulation amount is controlled by adjusting the aperture of the diaphragm and the shutter time, that is, the exposure time.

[0003]

In an image sensor, as the temperature increases, the value of the dark current generated in the photodiode increases. As the exposure time increases, the effect of the dark current becomes more pronounced, and the noise component generated in the captured image increases. Increase.
That is, for example, in shooting where long-time exposure is performed,
The influence of the dark current is large, and the image quality of the captured image is likely to deteriorate.

An object of the present invention is to provide an exposure time control device capable of reducing the influence of noise generated on a captured image due to a dark current and preventing the image quality of the captured image from deteriorating.

[0005]

An exposure time control device for an electronic still camera according to the present invention includes a temperature sensor for detecting a temperature of an image sensor, and a maximum exposure time in automatic exposure control, which is set to a value corresponding to the temperature. And a maximum exposure time control means for limiting.

[0006] Preferably, the temperature sensor measures the temperature of the surface of the image sensor. The temperature sensor measures, for example, the temperature of the back surface located on the opposite side of the light receiving surface of the image sensor.

It is preferable that the maximum exposure time control means shortens the maximum exposure time as the temperature of the image sensor increases. In this case, as an example, when the temperature is T ° C., the maximum exposure time at 0 ° C. is t ₀ , and when 7 ≦ Th ≦ 11,
The maximum exposure time at T ° C. is t _max = t ₀ × (1 /
2) ^{T / Th} .

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an electronic still camera provided with an exposure time control device according to an embodiment of the present invention.

This electronic still camera is a single-lens reflex camera, and an interchangeable lens 11 is electrically connected to an electric circuit provided in the camera body via mount pins 12 and 13. A front lens group 14 and a rear lens group 15 are provided in the lens barrel of the interchangeable lens 11.
An aperture 16 is provided between 4 and 15. The lenses 14 and 15 are displaced in the direction of the optical axis under the control of the lens control circuit 17 to perform focus adjustment. Lens control circuit 1
7 operates according to a control signal sent from the system controller 31 provided in the camera body via the mount pin 12. The iris 16 operates according to a control signal sent from the iris drive circuit 32 provided in the camera body via the mount pin 13, and the opening of the iris 16 is adjusted. The aperture drive circuit 32 is controlled by the system controller 31.

In the camera body, lenses 14, 15
A quick return mirror 21 is provided on the optical axis. The quick return mirror 21 is rotatable between an illustrated inclined state and an upwardly rotated horizontal state. A focusing plate 22 is provided above the quick return mirror 21, and a pentaprism 23 is provided above the focusing plate 22. Behind the pentaprism 23, an eyepiece 24 of a finder is provided.

Behind the quick return mirror 21,
A shutter 25 is provided, and an infrared cut filter 26 and an optical low-pass filter 27 are provided behind the shutter 25. Behind the optical low-pass filter 27, CC
A D (image sensor) 33 is provided. That is, the quick return mirror 21, shutter 25, infrared cut filter 26, optical low-pass filter 27, CCD 33
Are arranged on the optical axis of the lenses 14 and 15. In the vicinity of the CCD 33, a temperature sensor 60 for detecting the temperature of the CCD 33 is provided.

The rotating operation of the quick return mirror 21 is driven by a mirror driving circuit 34, and the opening and closing operation of the shutter 25 is driven by a shutter driving circuit 35.
The mirror drive circuit 34 and the shutter drive circuit 35 are controlled by the system controller 31.

Normally, the mirror 21 is set in an inclined state, and guides the light taken from the interchangeable lens 11 to the pentaprism 23 side. At this time, the shutter 25 is closed,
The optical path toward the CCD 33 is closed. On the other hand, when photographing is performed, the mirror 21 is rotated upward under the control of the mirror driving circuit 34, and becomes a horizontal state. With the rotation of the mirror 21, the shutter 25 is opened under the control of the shutter drive circuit 35, and the light taken in from the interchangeable lens 11 irradiates the light receiving surface of the CCD 33. That is, an image obtained by the lenses 14 and 15 is formed on the light receiving surface, and the CCD 33 generates an imaging signal corresponding to the image.

A pulse signal generating circuit (PPG) 36 is connected to the system controller 31, and the pulse signal generating circuit 36 generates various pulse signals under the control of the system controller 31. The CCD drive circuit 37, the A / D converter 38, and the image signal processing circuit 39 are driven based on these pulse signals, and the operation of the CCD 33 is controlled by the CCD drive circuit 37. That is, the imaging signal read from the CCD 33 is converted into a digital signal by the A / D converter 38, and is subjected to predetermined image processing in the image signal processing circuit 39. Image signal processing circuit 39
Is connected to a memory 40 having a sufficient capacity to store digital image data corresponding to one image.

The image signal processing circuit 39 includes a monitor interface 41, a card interface 42,
The C interface 43 is connected. These interfaces 41, 42, 43 are controlled by the system controller 31.

A backlight 45 and a liquid crystal display (LCD) 46 are connected to the monitor interface 41 via a liquid crystal drive circuit 44.
9, a video output terminal 50 is connected. The liquid crystal drive circuit 44 is controlled based on the image data read from the memory 40, and an image is displayed by the liquid crystal display element 46. The image data is supplied to the video output drive circuit 4
At 9, the data is converted into a predetermined format and output to an external video device via a video output terminal 50. A card connector 47 is connected to the card interface 42, and a PC connector 4 is connected to the PC interface 43.
8 are connected. An IC memory card can be attached to the card connector 47, and a personal computer can be connected to the PC connector.

An AF sensor 51 and a photometric sensor 52 are connected to the system controller 31. The AF sensor 51 has a conventionally known configuration, and the AF sensor 51 measures the focus adjustment state of the lenses 14 and 15. The opening degree of the aperture 16 during exposure and the CC
Photometry for determining the charge accumulation time (exposure time) in D33 is performed.

An operation switch 54 and a status display device 55 are connected to the system controller 31. The operation switch 54 includes a photometric switch, a release switch, and the like. The photometry switch is turned on by half-pressing a release button (not shown), whereby the photometry sensor 52 performs photometry. The release switch is turned on by fully depressing the release button, whereby the shutter 25 is driven to open and close. That is, the CCD 33 is exposed, and an imaging signal corresponding to an image is generated in the CCD 33. Status display device 5
Reference numeral 5 denotes a liquid crystal display element on which various setting states of the electronic still camera are displayed.

FIG. 2 shows an example of a mounting structure of the temperature sensor 60. The CCD 33 is disposed in parallel with the substrate 61, and terminals 62 extending from both sides of the CCD 33 are connected to the substrate 61.
It is fixed to the surface of. The temperature sensor 60 is a CCD3
3 is attached to the back surface 33 b located on the opposite side of the light receiving surface 33 a, that is, the surface facing the substrate 61. Substrate 61
An opening 63 for guiding cooling air around the CCD 33 is formed in a portion facing the temperature sensor 60.

FIG. 3 is a flowchart showing a photographing operation control routine in the electronic still camera. In step 101, it is determined whether or not the photometric switch is on. When the metering switch is turned on,
Step 102 is executed, and the temperature sensor 60
The temperature T ° C. of the CD 33 is measured. In step 103, the temperature T ° C. in the automatic exposure control is calculated according to the equation (1).
In this case, the maximum exposure time t _max , that is, the longest feasible exposure time is obtained. t _max = t ₀ × (１ ^／ ) ^{T / Th} (1) where t ₀ is the maximum exposure time at 0 ° C. and 7 ≦ Th ≦ 11. In the present embodiment, Th = 10, that is, t _max = t ₀ × (１ ^／ ) ^{T / 10} (1 ′) is actually used.

In step 104, an exposure calculation is performed. That is, the opening degree of the diaphragm 16 and the exposure time are calculated based on the photometric value obtained by the photometric sensor 52.
At this time, the exposure time is determined within a range not _{exceeding the} maximum exposure time _tmax . The exposure calculation will be described later in detail.

In step 105, it is determined whether or not the release switch has been set to the ON state. When it is determined that it is not in the ON state, the process returns to step 101,
When it is determined that the switch is on, the process proceeds to step 106. In step 106, the mirror 21 is moved up to the horizontal state, and the diaphragm 16 is moved to step 104.
Is determined by the calculated opening. Step 107
Then, the shutter 25 is opened, whereby the CCD 33
, Charge accumulation starts. In step 108,
It is determined whether the exposure time calculated in step 104 has elapsed. Step 1 when the exposure time elapses
From step 08, the process proceeds to step 109, where the shutter 25 is closed and the mirror 21 is rotated to the inclined state.

In step 110, the reading of the electric charge accumulated in the CCD 33, that is, the image pickup signal is started. In step 111, the read image signal is subjected to image processing such as interpolation, color correction, and gamma correction in the image signal processing circuit 39, and the data of the captured image is stored in the memory 40, and The operation control routine ends.

Next, the exposure calculation routine executed in step 104 will be described. A dark current is generated in the CCD 33 even when the light is completely blocked, and the magnitude of the dark current changes according to the temperature of the CCD 33. CCD33
FIG. 4 shows an example of the relationship between the temperature and the dark current. As understood from this figure, the value of the dark current at the temperature T ° C. is represented by Id
Assuming that the value of the dark current at a temperature of 0 ° C. is Id ₀ , the ratio of the dark current Id / Id ₀ is smaller than 1 at 0 ° C. or less,
The temperature rapidly increases as the temperature T ° C increases, and approximately doubles when the temperature T ° C increases by 8 to 10 ° C.

In the present embodiment, the maximum exposure time t _max is determined by equation (1 ′), and the ratio of the temperature to the maximum exposure time t _max
The relationship with _max / t ₀ is as shown in FIG. That is, as the temperature rises, the ratio t _max / t ₀ rapidly decreases.

FIGS. 6 and 7 are flowcharts showing an exposure calculation routine. In step 201, the maximum exposure time t _max is converted into an apex value Tv _min . That is, the apex value Tv decreases as the exposure time increases, and the maximum exposure time t _max corresponds to the minimum value of Tv. The conversion into the apex value is because the maximum exposure time t _max is represented by real time, while the aperture value and the shutter speed (exposure time) are calculated using the apex value. Similarly, in step 202, the sensitivity level of the CCD 33 (that is, the AGC level of the video signal) is converted into an apex value Sv.

In step 203, a photometric value Bv is obtained based on the data obtained from the photometric sensor 52.
In step 204, the Ev value is obtained according to the equation (2). Ev = Bv + Sv (2) In step 205, the Ev obtained in step 204 is substituted into the equation (3) indicating the slope portion of the program diagram, and Tv is obtained. Tv = (3/8) × Ev + 3 (3)

[0028] At step 206, whether Tv is greater than the maximum value Tv _max that is set in advance is determined. When Tv is greater than the maximum value Tv _max, the maximum value Tv _max is replaced as Tv at step 207. When Tv is equal to or less than the maximum value Tv _max, the step 207 is skipped. In step 208, (4)
Av is determined according to the equation. Av = Ev−Tv (4)

In step 210, it is determined whether Av is larger than a preset minimum value Av _min . When Av is larger than the minimum value Av _min , the process proceeds to step 211. When Av is equal to or smaller than the minimum value Av _min ,
Proceed to step 221.

At step 211, it is determined whether or not Av is larger than a preset maximum value Av _max .
When Av is greater than the maximum value Av _max, step 21
In 2, the maximum value Av _max is replaced as Av. In contrast, when Av is equal to or less than the maximum value Av _max, step 212 is skipped. Step 213
Then, Tv is obtained according to the equation (5). Tv = Ev−Av (5)

In step 214, Tv is set to the maximum value Tv.
_maxIt is determined whether it is greater than or equal to. Tv is the maximum value T
v_maxIs greater than the maximum in step 215
Value Tv _maxIs replaced as Tv. And step
In step 216, the status display device 55 (FIG. 1)
Tv is the maximum value Tv_maxIndicates that it was restricted by
In order for this routine to flash,
finish. That is, in this case, the exposure time is the maximum value Tv
_maxTo the minimum value (highest shutter speed) corresponding to
Limited. On the other hand, at step 214, Tv becomes Tv
_maxIf it is determined that the value is less than or equal to
This routine ends without executing step 16.

In step 221, the minimum value Av _min is set to A
v. In step 222, Tv is obtained according to equation (5). In step 223, Tv
Is smaller than the minimum value Tv _min . T
When v is smaller than the minimum value Tv _min , that is, when the exposure time corresponding to Tv is longer than the maximum exposure time t _max , the minimum value Tv _min is replaced with Tv in step 224. That is, the maximum exposure time t _max is set as the exposure time. In step 225, a predetermined mark blinks on the status display device 55 to indicate that Tv is limited by the minimum value Tv _min ,
This routine ends. In contrast, step 223
When it is determined that Tv is equal to or greater than Tv _max ,
This routine ends without performing steps 224 and 225.

As described above, in this embodiment, the CCD 33
Is controlled such that the maximum exposure time in the automatic exposure control becomes shorter according to the equation (1 ′) as the temperature of the camera becomes higher.
That is, when the value of the dark current generated in the photodiode of the CCD 33 increases, the maximum exposure time is limited so as to be shortened. Therefore, the amount of noise components generated in the captured image is suppressed, and the image quality is prevented from deteriorating. The maximum exposure time according to the temperature can be secured.

The maximum exposure time does not need to be calculated according to the equation (1 '). For example, the relationship between the temperature and the maximum exposure time is stored in the memory 40 in the form of a table, and the address corresponding to the temperature is referred to. Then, the maximum exposure time may be read out.

[0035]

As described above, according to the present invention, it is possible to reduce the influence of noise generated in a photographed image due to dark current, and to prevent the image quality of the photographed image from lowering.

[Brief description of the drawings]

FIG. 1 is a block diagram of an electronic still camera including an exposure control device according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing an example of a structure for mounting an image sensor of a temperature sensor.

FIG. 3 is a flowchart illustrating a shooting operation control routine.

FIG. 4 is a diagram illustrating an example of a relationship between temperature and dark current.

FIG. 5 is a diagram illustrating an example of a relationship between a temperature and a maximum exposure time.

FIG. 6 is a first half of a flowchart showing an exposure calculation routine.

FIG. 7 is a second half of a flowchart showing an exposure calculation routine.

[Explanation of symbols]

 33 CCD (imaging device) 60 Temperature sensor

Claims (5)

[Claims] 1. An electronic still camera comprising: a temperature sensor for detecting a temperature of an image sensor; and a maximum exposure time control means for limiting a maximum exposure time in automatic exposure control to a value corresponding to the temperature. Exposure time control device. 2. The exposure time control device according to claim 1, wherein the temperature sensor measures a temperature of a surface of the image sensor. 3. The exposure time control device according to claim 2, wherein the temperature sensor measures a temperature of a back surface opposite to a light receiving surface of the image sensor. 4. The exposure time control device according to claim 1, wherein the maximum exposure time control means shortens the maximum exposure time as the temperature increases. 5. When the temperature is T ° C. and the maximum exposure time at 0 ° C. is t ₀ , and when 7 ≦ Th ≦ 11, T ° C.
5. The exposure time control device according to claim 4, wherein the maximum exposure time at the time of is t _max = t ₀ × (）) ^{T / Th} .